THE MERCK VETERINARY MANUAL
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Overview of Reptiles

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The class Reptilia includes >8,000 species, but only a few dozen are likely to be encountered in general practice. All the Crocodilia, front-fanged poisonous snakes (but not all backed-fanged poisonous species) and both species of poisonous lizard (Heloderma spp) are considered to be dangerous animals and are usually covered by federal and/or state legislation. These species are not generally kept as pets and will therefore not be discussed here. The class Reptilia includes four orders: Crocodyla (crocodiles, alligators, gharials), Testudines (turtles and tortoises), Squamata (lizards and snakes), and Rhynchocephalia (tuataras).

Reptiles are vertebrates with organ systems similar to those of mammals. However, they are ectothermic and rely on environmental temperature and behavior to control their core body temperature. They possess both renal and hepatic portal circulations, and predominantly excrete ammonia, urea, or uric acid depending on their evolutionary adaptations. Their RBCs are nucleated, and their metabolic rates are lower than those of mammals. All reptiles exhibit ecdysis—a normal process by which the outer skin is periodically shed. Diurnal species require broad-spectrum light for vitamin D3 synthesis and calcium homeostasis. Fertilization is internal, and females may produce eggs (oviparous) or live young (ovoviviparous).

Reptiles are not considered highly social creatures, and multiple-male groups can lead to intraspecies aggression. Single-male, multiple-female groupings can work well for certain species, but the solitary reptile is often the healthiest pet. The lifespan of many reptiles can exceed 10–20 yr, requiring a longterm commitment from owners.

Reptiles possess a common cloaca, which receives the lower GI, reproductive, and urinary tracts. In addition, lungs are simpler and composed of vascular pockets, more like a cavitated sponge than alveoli.

Lizards and chelonians are quadrupeds and have a familiar pentadactyl limb arrangement. Reptiles lack a true diaphragm; in many species, all organs are contained within a single coelomic cavity. Some lizards (eg, tegus and monitors) have thin postpulmonary and/or post-hepatic membranes that divide the coelom into compartments. Snakes' organs are distributed in a longitudinal arrangement. Boas and pythons are primitive snakes and have both left and right lungs; however, other snakes lack a developed left lung. Squamates have incomplete tracheal rings, and males have paired copulatory organs (hemipenes).

The chelonians are characterized by their shell, which comprises a dorsal carapace and ventral plastron. The internal organs are separated by two thin membranes. The heart is located within a cardiac membrane, while the lungs are dorsad and separated from the remaining viscera by a postpulmonary membrane (or septum horizontale). Chelonians have complete tracheal rings, and males have a single copulatory phallus.

Reptiles rely on environmental temperature and behavior to maintain their body temperature within their preferred optimal temperature zone (POTZ). Within this species-specific POTZ, a reptile is able to achieve the preferred body temperature for specific metabolic activities, which may vary diurnally and seasonally and by age and gender. The metabolic rate of reptiles is lower than that of mammals and birds. Consequently the K constant to determine energy expenditure, nutritional requirements, and allometric drug doses is given by the equation BMR = K(body wt [in kg] 0.75), in which BMR = basal metabolic rate in Kcal/day and K = 10 (the energy constant for reptiles).

All snakes, lizards, and chelonians have a three-chambered heart (two atria and one ventricle), whereas crocodilians have a four-chambered heart. All reptiles have both pulmonary and systemic circulations (similar to mammals). In noncrocodilian reptiles, functional separation of venous and arterial blood is largely maintained via a muscular ridge within the ventricle. Peripheral blood cell types include erythrocytes, heterophils, eosinophils, basophils, lymphocytes, monocytes (including azurophils), and thrombocytes. Renal and hepatic portal circulations exist, and intra- and extracardiac vascular shunts may be present, especially in aquatic species.

Reptilian skin is usually heavily keratinized and protected by scales. The chelonian shell is composed of both dermal bone plates and keratinized epithelial scutes. Reptiles do not have extensive skin glands, and their skin is essentially dry. However, many male lizards have a series of pre-anal or femoral pores located cranial to the vent or along the craniomedial aspect of the hindlimbs. In some species, both sexes have these glands, but they are more pronounced in the male. Chromatophores are common and enable many species, most notably the Chamaeleonidae, to change color. Bony skin structures, osteoderms, are found in the crocodilians and some lizards. These structures may interfere with radiographic interpretations and surgery. Certain species of snakes have heat-sensitive receptors located around the maxilla that are used to locate prey. Skin characteristics (eg, crests, spines, dewlaps) are often used for species or gender identification in those species that exhibit dimorphic variation.

All reptiles shed their skin. The frequency of ecdysis depends on species, age, nutritional status, environmental temperature and humidity, reproductive status, parasite load, hormonal balance, bacterial/fungal skin disease, and skin damage. The entire process can take 7–14 days.

Most species require some form of conditioning before breeding (eg, hibernation, seasonal nocturnal cooling). Many species are sexually dimorphic. Male lizards are generally larger, have pre-anal or pre-femoral pores, have hemipenal bulges at the tail base, and often are more brightly colored. The gender of snakes is determined by probing for the hemipenes with a blunt, lubricated probe. In males, the probe will enter to a depth of 6–14 subcaudal scales, whereas in females it enters a cloacal gland only to a depth of 3–6 scales. Sexual dimorphism in chelonians is usually obvious in adults; males often have a concave plastron and a longer tail. Many reptiles, especially lizards and chelonians, are territorial and will fight conspecific males, causing severe injuries. In addition, overzealous and unrelenting males may ardently pursue females, causing repeated harassment. Fertilization is internal, and reproduction is either oviparous (egg production) or ovoviviparous (live bearing). Gender determination may be genetic (most snakes, many lizards) or related to incubation temperature (most chelonians, some lizards).

Last full review/revision October 2015 by Stephen J. Divers, BVetMed, DZooMed, DACZM, DECZM (herpetology), FRCVS

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